The present invention relates to method and apparatus for converting an image signal of a hivision TV to an image signal of a computer terminal, and more particularly to method and apparatus for converting an image signal effective in a superimpose apparatus which combines an image generated by a character terminal or a graphic terminal with an image generated by a standard hivision TV.
Combination of the image of the computer terminal and the image of the standard TV system such as NTSC system has been expected in various fields. Image resolutions of personal computers and work stations has been recently more and more increasing, and a non-interlace system having 700-800 dots horizontally and approximately 520 dots vertically has been becoming common. When the character image or graphic image (called terminal image hereafter) of such a terminal is to be combined with an image of the NTSC system TV, the following problems arise. In the NTSC system, (1) a scan line (dot) density per field is approximately one half of that of the terminal image, (2) signals between two adjacent fields have one-half scan line shift because of interlace scan (scan lines of first and second fields are shifted by one-half scan line interval), and (3) if a video signal is converted to a pixel density of the terminal image, an unnatural border line appears on an animation image (normally received image).
A difference between the dot densities of the NTSC system and the terminal image may be resolved by using a hivision signal which is a future TV signal system. The hivision TV signals has a specification of 1125 scan lines (interlaced scan), 20 MHz video band and 5/3 aspect ratio. The number of scan lines per field and the field frequency are essentially same as those of the terminal image.
In the combination of the hivision signal and the terminal image signal, the amounts of information are balanced as described above but the shift of one-half scan line interval between the adjacent field signals due to the interlaced scan system remains, and a difference between occupation times per dot is created due to the difference between aspect ratios. The difference between the occupation times per dot is due to the fact that the hivision signal and the terminal signal are of the same scan time in spite of the fact that a horizontal scan distance of the hivision signal is 5/4 of that of the terminal signal (assuming that an image height for the hivision signal is equal to that for the terminal signal). If the hivision signal is outputted to a display having and aspect ratio of 3/4, the image is vertically expanded. Accordingly, in the prior art as shown in JP-A-60-42993, where a clock time for reading and writing a field memory is changed, it is accomplished by using one field of memory of the hivision TV signal. As a result, a memory capacity is as large as approximately 2M bytes. In the prior art, the shift of information by one-half scan line interval between adjacent scan lines of the interlaced signal, which is one of the problems mentioned above, is not resolved. When the interlace system signal is converted to a non-interlace system signal, signals shifted by one-half scan line interval are displayed on the same scan position and hence the image vibrates (at a high contrast area) or becomes dim.
An example of conversion of the hivision TV to the NTSC system is disclosed in JP-A-59-104866.